1. Field of the Invention
The invention relates to an abnormality detection system and abnormality detection method for an internal combustion engine.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 2004-176719 (JP-A-2004-176719) describes an internal combustion engine in which a NOx selective catalytic reduction catalyst (hereinafter, referred to as “NOx SCR catalyst”) is arranged in an engine exhaust passage, and NOx contained in the exhaust gas is selectively reduced by ammonia that is generated from an aqueous urea solution that is supplied to the NOx SCR catalyst. According to JP-A-2004-176719, a sensor that detects, for example, ammonia generated from the aqueous urea solution is provided in the engine exhaust passage at a position downstream of the NOx SCR catalyst, and it is determined that an abnormality has occurred if a signal output from the sensor changes in an unexpected manner when the amount of aqueous urea solution that is supplied to the NOx SCR catalyst changes.
However, the signal output from the sensor changes in an unexpected manner in either the case where the amount or quality of aqueous urea solution that is supplied to the NOx SCR catalyst deviates from a regular value or the case where the NOx SCR catalyst deteriorates. Therefore, according to the above-described method, it is not possible to determine whether such an unexpected change has occurred due to an irregularity in the amount or quality of aqueous urea solution that is supplied to the NOx SCR catalyst or due to deterioration of the NOx SCR catalyst.